Just adding a message to clarify: I have installed PP on three Nuke sites, and have had to change the "URLs to your Forum Directory" to my Nuke ROOT directory.
Setting the URL to the actual forum directory gave me this incorrect Register link: http://www.MYSITE.com/modules/Forums...&mode=register
The correct link should be: http://www.MYSITE.com/modules.php?na...&mode=register
I.e., the error was caused by the entry in admin's Forum Settings area: http://www.MYSITE.com/modules/Forums
(the true Forums directory);
When I replaced this setting with the Nuke root, http://www.MYSITE.com/
, the error disappeared.
Bottom line: the fix for Nuke problems is to change the admin setting to the Nuke ROOT, not the Forums directory.